The present invention relates to electrical resistance heaters. More particularly, the present invention relates to an electrical cartridge heater assembly having improved electrical connection for the external lead wires.
Cartridge heaters typically comprise an electrical resistance element which is helically coiled on an insulating core. A metal cartridge or cannister that is coaxial with the coil and the core is radially spaced from the coil. The space between the cannister and the coil is filled with a mineral insulating material which has an optimum combination of a low electrical conductivity and a high thermal conductivity. In conventional cartridge heaters the mineral insulating material is typically a particulate material, such as a granular or powdered magnesium oxide, which is confined within the cannister by an insulating end plug.
Electrical power is delivered to the heater through a pair of conductors which are contained within longitudinal bores in the insulating core and are electrically connected to the resistance element. The conductors terminate in external lead pins which extend from the core outwardly through the end plug of the electrical heater. The end terminals of the helically coiled resistance element extend into the longitudinal bores in the core where they are held in electrical contact with the conductors. Opposite ends of the helically coiled resistance element are connected to different conductors so that the electrical power can be supplied to the coil. The lead pins which extend from the outer end of the conductors are conventionally solid as opposed to being of stranded wire in order to minimize surface area and thereby avoid the detrimental affects of surface oxidation. The use of solid conductors with terminating solid lead pins also makes the cartridge heater assembly reasonably rigid and provides for easy centering of the internal elements so that the assembly can be more easily fabricated.
In typical cartridge heaters, each lead pin extends outwardly from the terminal end of the heater with the outward extension of the lead pin being adapted to be electrically connected to an external lead, which is typically a flexible lead wire. Because the lead pins are generally lengthy, the electrical connections between the lead pins and the external lead wires are typically enclosed in an extension which occupies a relatively large amount of space at the end of the heater, thereby adding to the unheated length of the cartridge heater assembly. Such cartridge heater assemblies having a large unheated length on the terminal end are exemplified by U.S. Pat. No. 3,839,623 to Portmann; U.S. Pat. No. 4,287,502 to Nickmeyer; and U.S. Pat. No. 4,622,455 to Schwarzkopf.
This additional unheated length is undesirable because it acts as a heat sink, causes an uneven heat flux and temperature distribution along the length of the heater, reduces the wattage output of the heater in those applications where the length of the heater is limited, and prevents use of the heater in certain applications where space limitations are tight.
In U.S. Pat. No. 4,346,287 to Desloge, the inventor seeks to avoid the disadvantages of the prior art by providing the connections between the heater lead pins and the flexible external lead wires internally within the insulating end plug of the cartridge heater. In order to accomplish this, the lead pin and the insulating end plug are drilled out to provide cylindrical bores into which the terminal ends of the lead wires are inserted for a lapped connection. The primary problem with this concept is that it requires high precision drilling in order to provide a concave surface in each lead pin, which is, of course, expensive. Moreover, the inventor discloses numerous embodiments wherein he does, in fact, add additional enclosures and extensions over the terminal connection of the lead pins and the external lead wires, which thus defeats his original purpose of eliminating the additional unheated length to the cartridge heater.
With this then being the state of the art, it is an object of the present invention to provide an electrical heater assembly having secure terminal connections between the heater lead pins and the terminal ends of the external lead wires.
It is another object of the present invention to provide such secured connections within an end assembly which minimizes the amount of unheated length which is added to the terminal end of the electrical heater assembly.
It is a further object of the present invention to provide the foregoing objects without the additional expense of high precision machine operations.
These and other objects of the present invention, as well as the advantages thereof, will become more clear from the disclosure which follows.